public class Solution4 {
    public long prev = Long.MIN_VALUE;
    public boolean isValidBST(TreeNode root) {
        if(root == null) {
            return true;
        }
        boolean left = isValidBST(root.left);
        // 剪枝
        if(left == false) {
            return false;
        }
        boolean cur = false;
        if(root.val > prev) {
            cur = true;
        }
        // 剪枝
        if(cur == false) {
            return false;
        }
        prev = root.val;
        boolean right = isValidBST(root.right);
        return left && cur && right;
    }
}